Free-living Flatworms with fascinating egg cases
When I go for a walk, I have a habit to always take along some small plastic bag or a glass jar in case I stumble across something interesting that I would like to examine more closely at home or later on in the lab. It was this habit of mine that got me into collecting and keeping some terrestrial flatworms, known also as land planarians. Most of these unsegmented wormy creatures that possess only one body opening which functions as both mouth and anus (whichever happens to be more appropriate at a given time and situation) are aquatic.
However, some -especially species occurring in tropical or subtropical humid places- are terrestrial and the first ones I collected on Hachijojima, where I spent four and a half interesting years, were indeed those terrestrial ones. At night and after some rain one may see one crawl, actually seemingly sliding with the help of minute cilia on their skin, along the road surface in search of some shelter like the underside of a log or boulder. Safe from predators (although there aren’t really many), but near their favourite food, i.e. earthworms, the planarian flatworm establishes its home. It appears to have chemoreceptors that inform it of the presence of an earthworm and it then stalks its prey. If the planarian brushes against an earthworm, the latter reacts as if mortally scared and tries to get away as quickly the earthworm can. But often it is to no avail and the writhing and twisting of its body will not free it from being sucked up by the planarian flatworm.
Planarians are fascinating creatures and famous for being able to grow back into adult worms even if cut into as many as a hundred pieces. I collected 5 different species, some quite beautiful with reddish markings like some 3-4 cm long species and some others almost 40 cm long with many parallel blackish lines all along their body. But none of them had unambiguously been taxonomically described and identified. In addition to the mentioned asexual reproduction by fragmentation or fission, many species, being hermaphrodites, also lay eggs or, to be more precise, deposit egg cases containing 1 to 6 eggs. In autumn one of my two large black individuals and one of the only 3-4 cm long species produced three egg cases, which were initially bright red in colour when freshly laid on the moist soil but the turned a rather greyblack within 5 hours after laying. I had these three roughly pea-sized dark egg cases and wondered about the eggcase’s ‘shell’ after two white 1 cm long baby worms had hatched about 3 weeks later from one of the egg cases: the thin and supple shell had to be tough and prevent desiccation, but needed to allow oxygen to get in and CO2 to get out.
With my colleague at Oulu University in Finland, we then examined the eggcase’s shell by scanning electron microscopy and at first were disappointed: no special features at all on the outer and inner surface of the shell were to be seen. However, to our great surprise, we found arrays of beautifully regular and parallel nano-tubes under the smooth shell linking inner and outer shell surfaces. And what was even more surprising, these nanotubes were of the same dimensions (i.e., 120 nm in diameter) and very similar in chemical composition with regard to their amino acid content to the teeth of the sucker organs of squid tentacles! The similarity was amazing and at the same time, hugely puzzling as planarian worms and squid (which are classified as molluscs) have absolutely nothing in common evolutionarily. Or do they have perhaps? Perhaps the similarity between the structure of the planarian egg case and the squid’s sucker teeth could be a case of convergent evolution. In that case, we must look for a functional reason why a seemingly identical structural organization manifested itself in two different situations. After all, both have to be tough and flexible. Or could it be simply a kind of co-incidence? We are still working on this problem and therefore a conclusion as to what might have been the cause for this puzzling similarity is still unavailable. Cases like these where one finding leads to further questions are common in science, in fact, are part of the fascination with science. So, keep posted and find out if we can come up at all with a reasonable explanation in the future.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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